Process for the preparation of high alpha cellulose fiber for conversion into cellulose derivatives



' No Drawing.

Patented May 2, 1933 UNITED STATES PATENT OFFICE.

GEORGE A. RICHTER, OF BERLIN, NEW HAMPSHIRE, ASSIGNOR TO BROWN COMPANY; OF BERLIN, NEW HAMPSHIRE, A CORPORATION OF MAINE PROCESS FOR THE PREPARATION OF HIGH ALPHA CELLULOSE FIBER FOR CONVERSION INTO GELLULOSE DERIVATIVES This invention relates to the production from raw or unbleached pulp of a fiber having an especially high alpha cellulose content and which is particularly qualified for conversion into cellulose esters, such as viscose, cellulose acetates, cellulose nitrates, and other cellulose derivatives. The fiber herein contemplated, however, because of its low strength and tear test qualities as a result of'the special process by which it is prepared, is not especially suitable for papermaking, but may, if desired, be employed as a filler or raw material in a papermaking or felt-making furnish or for other purposes where its high absorbency and other characteristics are of value.

In order -to more fully understand the present invention and the results accomplished thereby, it is desirable to consider briefly certain investigations which I have made in connection with the purification of pulp to a high alpha cellulose content with alkaline liquors, and to scrutinize the results obtained from these investigations. Thus, if the usual bleached sulphite fiber is treated at .room temperature with a solution of caustic soda of about 17.5% to 18% concentration for thirty minutes, is then washed free of alkaline liquor, and, after acidification to 515 ght acidity is, weighed, the quantity of fiber obtained usually amounts to about 85% of the fiber initially subjected to treatment. The fiber which resists the action of the strong alkaline liquor, i. e., the amount of undissolved fiber obtained by a treatment of pulp under the specific conditions hereinbefore outlined, is called its alpha or resistant cellu lose content. This method of alpha cellu lose determination is empirocal, and in some instances may be carried out under different conditions to give a somewhat different determination of alpha cellulose content. For instance, if a more strongly alkaline liquor is employed in the treatment, or if the treatment is carried out at higher temperature or for a longer period of time, the alpha cellulose content obtained in such cases will be lower because of the more drastic reaction upon the fiber, resulting in the solution of more constituent therefrom. In fact, it is Serial No. 77,624.

possible to dissolve all the cellulose if the reaction on the pulp is made sufiiciently drastic.

It is obvious, therefore, that the fiber obtained from the standard method of alpha cellulose determination may not resist a second treatment with an 18% caustic soda solution. Thus, if the fiber obtained from the first alkaline treatment is re-treated With a freshly prepared 18% caustic soda solution under the standard conditions of alpha cellulose determination hereinbefore outlined, the results do not indicate that the fiber contains 100% alpha cellulose, but, on the contrary, show an alpha cellulose content of about 92% or even less. This signifies that the second treatment has effected a solution of certain constituents of the pulp which had resisted the previous alkaline treatment. If the fiber residuum from the second treatment is treated for a third time under the standard conditions with a freshly prepared caustic soda solution of 18% strength, the results indicate a fiber of about 95% alpha cellulose content. At each treatment of the bleached fiber with an alkaline liquor, more constituent is dissolved therefrom by such treatment. Moreover, not only is a solution of the less-resistant celluloses, ligneous, and other non-alpha cellulose constituents of the fiber effected, but also a progressive decomposition or degradation and solution of its alpha cellulose content.

If a series of successive alkaline treatments similar to those described is effected on an unbleached sulphite pulp, rather than on a bleached pulp, the important result to be observed is that the unbleached pulp appears to be pronouncedly more resistant to the dissolving action exercised thereupon by such successive treatments. The fiber from the initial alkaline treatment shows an alpha cellulose content of about 96% to 97%, and the fiber residuum from the second treatment shows an alpha cellulose content which amounts to about 97% to 98%. Further alkaline treatment does not appreciably alter this alpha cellulose content. The conclusion which may be drawn from these facts is, that a pulp which has been bleached is much more susceptible to degradation by an alkaline treatment or successive alkaline treatments such as hereinbefore described and that the result is not only a removal of non-alpha cellulose constituents of the pulp, but also in the degradation and removal of a portion of its alpha cellulose content. With this in mind, it is clear that in the production of a high alpha cellulose fiber, a process which comprehends the treatment or purification of a substantially unbleached pulp with an alkaline liquor should preferably be used.

In the manufacture of viscose, the initial step of the process consists in impregnating the bleached cellulosic material with a solution of caustic soda. Where a bleached pulp such as sulphite is employed as the raw material, a mass of fibers (usually in sheeted form) is soaked in a caustic soda solution of about 18% strength, and the excess solution contained in the sheet is expressed therefrom. The sheet is then shredded for treatment with the amount of carbon bisulphide necessary to form viscose.

The soaking of the mass of bleached pulp in a concentrated caustic soda solution e fects a solution of certain constituents contained in the mass. In other words, the conditions in the soaking treatment approximate those maintained in the procedure outlined for determining or analyzing the alpha cellulose content of cellulosic fiber. After the excess caustic soda has been expressed from the sheeted mass, the mass still contains a substantial amount of undissolved nonalpha cellulose constituent, which would respond to removal or solution by a second alkaline treatment, if such treatment were provided. In addition to the non-alpha cellulose constituents undissolved by the alkaline treatment, a certain amount of dissolved nonalpha cellulose constituent is contained in the caustic soda solution with which the sheeted mass is impregnated.

From the foregoing considerations, it should be apparent that there may be a marked difference in alpha cellulose content of various batches of fiber in a viscose plant immediately after the soaking and pressing operations, depending on the type and uniformity of the raw cellulosic material utilized,a non-uniformity in the pressed batches of fiber resulting in a non-uniform artificial silk product. For instance, if an over-bleached fiber having an initial alpha cellulose content of approximately is utilized as a raw material, the fiber resulting from the pressing operation will be inferior to, and will contain more non-alpha cellulose constituent than, a less drastically bleached fiber at the same stage of viscose manufacture. This condition of affairs, so far as I am aware has not previously been recognized, as it has generally been believed that the immersion or soaking of a pulp in a. bath of caustic soda solution prior to the conversion of such pulp into viscose, results in a purification or refinement of the pulp to a certain degree of purity or alpha cellulose content. The composition or particular type of pulp employed as the raw material was regarded as immaterial.

In accordance with the present invention, however, I preferably utilize unbleached pulp as a raw material, and purify such pulp to a high degree of purity prior to its conversion into viscose or other cellulose derivatives. The purification is carried out in a treating liquor of mercerizing strength. Unbleached pulp (sulphite pulp, for example) is treated approximately at room temperature with a caustic soda solution of about 18% strength, and after being washed free from solution and reaction products, as previously described yields a fiber containing about 96% to 98% alpha cellulose, with a yield of about 86% to 88%, based on the unbleached pulp employed. The purified fiber is then bleached with about 2% to 3% bleach, without materially afi'ecting its alpha cellulose content. A solution of such bleached purified fiber is of especially low viscosity. The fiber is very absorbent, has a very low pentosan content (usually amounting to less than 1%), and a very low ash content. The alkaline treatment in the cold, however, destroys the strength and tear test of the fiber, and such fiber cannot be hydrated to a slimy mass by a beating operation and is hence not suitable for papermaking when rattly, hard papers of high strength and tear resistance are in view. As hereinbefore indicated, however, the pulp is of value in some papermaking or felt-making furnishes.

In certain instances, where the initialv or raw pulp is of a refractory nature, that is, contains a higher percentage of non-alpha cellulose constituent as a result of undercooking in the production of the raw or unbleached pulp, or owing to the characteristics of the wood chips or the other raw cellulosic material from which such unbleached pulp is obtained, it may be desirable to treat the pulp prior to the alkaline purification, with a lignin-reactive reagent as in the form of a chlorinating or oxidizing liquor. This may be accomplished without materially affecting the yield or quality of the final fiber. The pulp is pretreated, in such instance, at about room temperature, in a liquor containing about 2% chlorine, or about 2% to 5% lime bleach, based on the dry Weight of pulp, for about twenty minutes to one hour. This pretreatment efiects the removal of certain nonalpha cellulose constituents, and conditions the pulp for subsequent alkaline treatment by forming reaction products with non-alpha rellulose constituents, which are more susceptible to removal by the alkaline treatment.

Such pretreatment, moreover, makes available for the production of a high alpha cellulose fiber, such as herein desired, certain raw pulps, such as Well-cooked kraft and soda pulps, which do not respond to an alkaline treatment alone. For instance, kraft pulp may be pretreated with a liquor containing about to lime bleach, or about 1% to 3% chlorine, prior to the alkaline treatment. In addition, such pretreatment makes possible the use of less bleach in the final bleaching step in producing a purified fiber of the desired whiteness. As a result, the bleached fiber is of higher alpha cellulose content, and is more rugged or resistant to any subsequent alkaline treatment.

cellulose content, as previously indicated, and

yields solutions of exceptionally low Viscosity. The combination of low viscosity and high alpha cellulose'content in fiber is unusual and highly advantageous. The caustic soda solution employed for the purification of the raw fiber may be of lower than 17% to 18% strength, when such purification may be carried out to aless extent. A fiber of about 95% alpha cellulose, for example, may be produced by treating unbleached sulphite pulp at about room temperature, in a caustic soda solution of about 10% strength, which is of mercerizing activity, although not as powerfully mercerizing as a caustic soda solution of 18% or greater strength. In any case, however, in order to produce a fiber having the .percentage of alpha cellulose herein desired, the

treatment of the unbleached pulp must take place in a solution of mercerizing activity, that is, one whose activity corresponds to a caustic soda solution of greater than 5% to 6% concentration at room temperature. The lower the temperature at which the treatment is effected, the lower the concentration of caustic soda necessary to eflect the desired purification or increase in alpha cellulose content of the pulp and the greater the tendency toward the destruction of the hydrating qualities of the pulp owing to the mercerization of the fiber, which is also favored at low temperature. This observation is similar to the principle Well known in the textile industry, that mercerization of cotton with a caustic soda solution is favored in the cold.

It may be desirable, in certain cases, to replace a portion of the caustic. soda of the alkaline treating liquor employed in the purification of the unbleached pulp by an equivalent amount of sodium sulphide or other alkaline compounds suitable for, the purification of such pulp. The use of a mixture of caustic soda and sodium sulphide as the purifying reagents may be particularly advantageous in an instance where a kraft mill is operated in close proximity to the raw pulp purification plant. The strong white liquor employed for digesting wood chips may be diverted from the kraft plant and employed as such, or mixed together with other alkaline liquor, for the purification of the raw pulp.

A process such as herein described, since it does not require the employment of steam or heat and results in a maximum yield of fiber of very high alpha cellulose content from the raw pulp, is very economical. Furthermore, substantially all the liquor separated from the purified fiber by washing, as hereinbefore described, may be re-employed for the treatment of other raw fiber, and the relatively small portion of alkaline compounds exhausted in the purification may be regenerated by subjecting a relatively small portion of the separated liquor to a recovery operation.

The purified fiber produced herein is very low in pentosans, having a value below 1, as compared with a value of about 2 to 2.5 possessed by high alpha cellulose fiber prepared by other processes wherein the hydrating qualities of the original raw fiber are preserved. As previously indicated, a further valuable property is the exceedingly low viscosity of solutions prepared therefrom.

I present herein a table showing the socalled cuprammonium viscosity (i. e., the viscosity of a solution of fiber in a liquor of standard strength comprising copper hydroxide and strong ammonia) of solutions prepared from various standard fibers:

From the foregoing table, it may be seen that with a given amount of cellulose in solution, a solution of the high alpha cellulose fiber produced by the method herein described is of materially lowerviscosity than that of solutions of the standard fibers. Consequently, when solutions of cellulose or cellulose esters are employed in operations or processes where a high concentration of the cellulose together with a low viscosity is desired, and where the concentration to be employed is limited by a viscosity of the solution obtained, the high alpha cellulose fiber herein produced may be employed at a much higher concentration and with more advantageous result than other standard fibers.

The term solution viscosity as hereinbefore applied to cellulose fiber is an arbitrary one, being indicative of the viscosity of a cellulose derivative solution preparable therefrom. The solution usually employed as a standard is a cuprammonium cellulose solution of prescribed cellulose concentration, the viscosity being determined by measuring the time of efllux of a definite volume of such solution under standard conditions, through an orifice of standard size. The solution viscosity of fiber is hereinbefore given in absolute c. g. s. units, and is determined by measuring the viscosity of a solution of 6 grams of fiber in a cuprammonium solution composed of 225 cc. of 28% ammonia water containing 9 grams of cuprous oxide. The c. g. s. unit is employed because it is definite, denoting a viscosity 100 times that of water at 20 C., where-fore a cupraminonium cellulose solution of standard composition identifying a fiber as having a solution viscosity of 10 is 100 times as viscous as water at 20 C. (ilycerine, which is often referred to when dealing with the solution viscosity of cellulose fiber, for example, has a value of between 8 and 10 units.

The fiber herein produced, moreover, is very absorbent, and thus absorbs caustic soda solution and other reagents very readily. Because of its especially high alpha cellulose content, when it is soaked in a solution of caustic soda for the first step of viscose manufacture, a smaller amount of caustic soda is consumed than ordinarily. When the excess caustic soda solution is expressed from the fiber, a product which is uniformly high in alpha cellulose content is obtained. The fiber herein produced is also especially suitable for conversion into other cellulose esters, such as cellulose acetate and the cellulose nitrates, as well as other cellulose derivatives.

I do not herein claim specifically the subject matter of application, Serial No. 140,677,

filed October 9,1926, by Milton 0. Schur, Royal H. Rasch, and myself, which discloses the advantages of mercerizing a bleached high alpha cellulose wood fiber prior to conversion into cellulose derivatives, and more particularly the nitrocelluloses.

Having thus described this invention, it should be obvious that it is susceptible of various changes and modifications without departing from its spirit or scope as defined by the appended claims.

4 What I claim is:

1. A process which comprises purifying cellulosepulp in a mercerizing liquor, and then bleaching the mercerized pulp.

2. A process which comprises purifying preliberated chemical wood pulp in a mercerizing liquor, bleaching the pulp, and converting the pulp into cellulose derivatives.

3. A process which comprises purifying substantially unbleached, cellulose pulp in a mercerizing liquor, and then bleaching the ulp.

p 4. A process which comprises purifying substantially unbleached chemical wood pulp in a mercerizing liquor, and then bleaching the pulp.

5. A process which comprises purifying substantially unbleached cellulose pulp in a mercerizing liquor, bleaching the mercerized pulp, and converting the bleached, mercerized pulp into cellulose derivatives.

6. A process which comprises purifying substantially unbleached chemical wood pulp in a mercerizing liquor, bleaching the mercerized pulp, and converting the bleached, mercerized pulp into cellulose derivatives.

7. A cellulosic material for use in making derivatives, said cellulosic material containing about 96% to 98% alpha cellulose and being capable of forming a solution having a viscositynot exceeding 2.

8. A cellulosic material for use in making viscose, said cellulosic material comprising alkali cellulose made from cellulose containing about 96% to 98% alpha cellulose, said cellulose being capable of forming a solution having a viscosity not exceeding 2.

9. A wood pulp especially adapted for use in making derivatives, said pulp containing at least about 93% alpha cellulose and being capable of forming a solution having a viscosity not exceeding 2.

In testimony whereof I have aifixed my signature.

GEORGE A. RICHTER. 

